1. Field of the Invention
The invention generally relates to a multi-input operational amplifier, and more particularly, to an output error compensation method of a multi-input operational amplifier.
2. Description of Related Art
In order to reduce the layout areas, many display driver integrated circuits (ICs) adopt multi-input operational amplifiers as their output buffers. A multi-input operational amplifier can generate a combined output voltage by interpolating several different voltages.
FIG. 1 is an internal circuit diagram of an input stage of a conventional multi-input operational amplifier. Referring to FIG. 1, the input stage of the multi-input operational amplifier has K differential pairs, wherein K is a positive integer greater than 1 (for example, 4). An input terminal of each of the differential pairs is coupled to an output terminal of the multi-input operational amplifier for receiving an output voltage VO, and the other input terminals IN1-INK of the differential pairs respectively receive input voltages V1-VK. Each of the differential pairs generates a differential signal V(+) and V(−) according to the received input voltage, such that a subsequent output stage circuit can generate the output voltage VO.
Assuming that the input voltages V1-VK contain only two voltages VH and VL, when the input voltages V1-VK are respectively VH, VL, VL, . . . , and VL, the output voltage VO at the output terminal of the multi-input operational amplifier 100 is equal to VL+(VH−VL)/K. Accordingly, the relationship between the input voltages V1-VK and the output voltage VO is obtained as shown in following table 1:
TABLE 1Input VoltageCombination TypeV1V2V3. . .VKVO1VLVLVL. . .VLVL 2VHVLVL. . .VL  VL  +            1      K        ⁢          (              VH        -        VL            )       3VHVHVL. . .VL  VL  +            2      K        ⁢          (              VH        -        VL            )       4VHVHVH. . .VL  VL  +            3      K        ⁢          (              VH        -        VL            )       5VHVHVH. . .VHVH
Theoretically, the output voltage VO can also be expressed in following expression 1:
                    VO        =                              (                                                            Ngm                  H                                ⁢                VH                            +                                                Ngm                  L                                ⁢                VL                                      )                                (                                          Ngm                H                            +                              Mgm                L                                      )                                              (        1        )            wherein N and M are respectively the numbers of the voltages VL and VH among the input voltages V1-VK received by the input terminals IN1-INK, and gmH and gmL are respectively the transconductances when the input voltages V1-VK received by the differential pairs are at the voltage levels VL and VH.
It can be understood based on foregoing expression (1) that the output voltage VO of the multi-input operational amplifier in FIG. 1 can be as ideal as that shown in table 1 only when gmH is equal to gmL. However, according to the relationship between transconductance and the input voltage difference±V on two input terminals of a differential pair (as shown in FIG. 2), the transconductance changes with the input voltage difference V. Thus, gmH and gmL may not be equal to each other. The difference between gmH and gmL produces an error in the output voltage VO of the multi-input operational amplifier 100 with a specific combination of the input voltages V1-VK, and accordingly the output voltage VO cannot be as ideal as that shown in foregoing table 1. For example, when K=4, the actual value of the output voltage VO with the input voltage combination type 2 (i.e., V1-V4 are VH, VL, VL, and VL) is lower than the ideal value
  VL  +            1      4        ⁢          (              VH        -        VL            )      thereof, and the actual value of the output voltage VO with the input voltage combination type 4 (i.e., V1-V4 are VH, VH, VH, and VL) are higher than the ideal value
  VL  +            3      4        ⁢          (              VH        -        VL            )      thereof.